The present invention relates to a method to transport a reference clock as described in the preamble of claim 1, to a method to synchronize a plurality of line terminals and to a method to synchronize a first line terminal in a first carrier unit with a second line terminal in a second carrier unit using such above method, as described in the preamble of, respectively, claim 3 and claim 4, and to a central office and a carrier unit, realizing such a method as described in the preamble of claim 5 and claim 6, and to a telecommunication network comprising such a central office and such a carrier unit as described in the preamble of claim 7.
Such a method to transport a reference clock signal from a central office to a line terminal in a communication system wherein the central office communicates with the line terminal in a carrier unit via an optical link is already known in the art, e.g. from the ITU-T Recommendation G.983.1 (10/98) with title xe2x80x98Broadband optical access systems based on Passive Optical Networks (PON)xe2x80x99. According to paragraph 8.3.5 xe2x80x98Transport specific TC functionsxe2x80x99 and more particular in paragraph 8.3.5.3 xe2x80x98Downstream PLOAM structurexe2x80x99 whereof the sub-paragraph 8.3.5.3.4 xe2x80x98Synchronization field (SYNC1-SYNC2)xe2x80x99 describes the purpose of the synchronization field SYNC1 and SYNC2 of the Physical layer Operation and Maintenance cell, shortly called hereafter PLOAM cell.
Such a physical layer operation and maintenance cell is a predefined part of one of the downstream information packets that is broadcasted on a predefined regular base from the central office to a plurality of line terminals in order to transport operation and maintenance information packets related to operation and maintenance functions.
Such functions are e.g.:
Performance monitoring;
Defect and failure detection;
System protection;
Failure or performance information;
Fault localization.
An optional Synchronization field of the PLOAM cell is described in the above-mentioned paragraph. The purpose of this field is to transport a 1 kHz reference signal provided at the optical line terminal OLT to the Optical Network Units ONU""s.
It has to be remarked that the OLT and the ONU""s are called hereafter and in the present claims, respectively, Central Office CO and Line Terminal LT.
As it is described in the Recommendation, a counter in the central office is incremented and reset on a regular base in order to generate a reference signal. At the central office, the value of that counter is taken right before transmission of the first PLOAM cell of a frame. The 15 least significant bits of the counter are placed in the 15 least significant bits of the SYNC1-SYNC2 field of the PLOAM cell. The most significant bit of the counter is placed in the most significant bit of the SYNC1 filed. Depending on the length of the counter other timing references can be obtained. At reception in the line terminal, this field is used to synchronize a local counter. The line terminal is then locked on the central office counter.
Thus a known method to transport a reference clock signal from a central office to a line terminal in e.g. a carrier unit via an optical link PON e.g. a PON system comprises the following steps:
a. retrieving the reference clock signal by the central office; and
b. synchronizing a first counter into the central office with the reference clock signal; and
c. regularly transferring counter values of the first counter as part of physical layer operation and maintenance cells over the optical link; and
d. adjusting a counter value of a second counter of the carrier unit when a counter value is received as part of a the physical layer operation and maintenance cell; and
e. deriving clock signals from the second counter for the line terminal.
A problem with this method is that the achieved accuracy is limited by two bytes i.e. the two predefined bytes SYNC1 and SYNC2 of each PLOAM cell in order to transport the reference clock. Indeed the number of bits that are representing the counter values of the first counter which are regularly transported are limited by the two predefined synchronization fields SYNC1 and SYNC2.
An object of the present invention is to provide a method to transport a reference clock signal from a central office to a line terminal in a communication system according to the above known method but wherein an improved accuracy is reached.
According to the invention, this object is achieved with the method to transport a reference clock according to claim 1, and with the method to synchronize a plurality of line terminals in a communication system and with the method to synchronize a first line terminal in a first carrier unit with a second line terminal in a second carrier unit that comprises such a method, according to, respectively, claim 3 and claim 4, and with the central office and the carrier unit that realizes such a method according to, respectively, claim 5 and claim 6, and with the telecommunication method according to claim 7, that comprises such a central office and such a carrier unit.
Indeed, by realizing the above mentioned step c) by means of assigning a predefined value to a predefined message identifier filed of the physical layer operation and maintenance cell and by comprising the counter values of the first counter in a plurality of predefined message fields of the physical layer operation and maintenance cell and by realizing the above mentioned step d) by means of recognizing the predefined value of the predefined message identifier field and by, upon recognition, extracting the counter value from the plurality of predefined message fields, a counter value represented by a bigger number as two bytes can be transported from the central office to the line terminal.
It has to be explained that the above mentioned Recommendation G.983.1 defines in paragraph 8.3.5.3.7 messages in the PLOAM cells to transport e.g. all Operation and Maintenance related alarms or threshold-crossing. Also all ranging related messages are mapped in the message field of the PLOAM cell. The format of the PLOAM message is indicated in Table 11/G.983.1. Herein it is shown that a message identifier field indicates the type of message and a number of message fields are containing the message. Furthermore, it is shown in Table 17 of this Recommendation, which defines the messages, that message number 20 with message name xe2x80x98Vendor specific messagexe2x80x99 describes a number of values for the message identifier field of the physical layer and operation cell, that are reserved for vendor specific messages from the central office to the line terminal. By assigning one of these vendor specific values to this predefined message identifier field it is allowed to comprise a vendor specific message in the message fields. According to the present invention the central office and the respective carrier units are instructed to associate the presence of a vendor specific value for the message identifier field with the transport of first counter value into a plurality of these message fields for synchronization purposes. Since the PLOAM cell contains up to 10 message fields an improved accuracy is realized with counter values of the first counter being represented by at least two bytes and by comprising these counter values in at least two of the predefined message fields.
Furthermore, when a counter value is received as part of physical layer and operation and maintenance cell by a carrier unit, the above mentioned step d) is realized i.e. adjusting a counter value of a second counter, by recognizing the predefined vendor specific value in the predefined message identifier field and by extracting the accurate counter value from the plurality of predefined message fields.
The method of the invention can be used to synchronize a plurality of line terminals in a communication system wherein a central office communicates with the plurality of line terminals in a carrier unit via an optical line. Indeed, the method comprises the step of transporting a reference clock signal according to the method of the present invention from the central office to each one of this plurality of line terminals. Hereby comprises step e) deriving from the second counter clock signals for each one of the plurality of line terminals whereby the line terminals are synchronize to each other. This is described in claim 3.
Another use of the present invention is realized by a method to synchronize a first line terminal in a first carrier unit with a second line terminal in a second carrier unit, in a communication system wherein a central office communicates with the first line terminal and with the second line terminal via, respectively, a first optical line and a second optical link.
Such a method is known in the art e.g. from the International Application published under the Patent Cooperation Treaty PCT at Jan. 7, 1999, with International Publication Number WO 99/00928 and with the title xe2x80x98Arrangement for synchronization of nodes in VDSL Systemsxe2x80x99. Therein an arrangement for synchronization of nodes in VDSLxe2x80x94systems, or more exactly, synchronization of optical nodes which share a common part of a cable in the access network between the subscribers and the local station are described. A time-synchronization towards an external system, for instance GPS, which gives a time reference by which the different nodes can be synchronized is provided. The synchronization reduces the near cross talk between the VDSL-system in the different nodes. Preferably respective node includes a receiver for a synchronization signal and an internal oscillator with high stability to deliver a stable clock signal.
This above known method to synchronize the first line terminal in the first carrier unit with the second terminal in the second carrier unit describes a step of synchronizing the first line terminal and the second line terminal to a common reference clock signal. By using the method of transporting a reference clock signal from the central office to the first line terminal according to the present invention and by using also the method of transporting a reference clock signal from the central office to the second line terminal according to the present invention and by constituting the reference signal each time with the common reference clock, both line terminals are synchronized to each other in a very accurate way. This is described in claim 4.
A preferred implementation of step a) of the method according to the present invention is described in claim 2. Indeed, by receiving during this step a) a universal clock signal and by constituting the reference clock with this universal clock signal a high clock accuracy is achieved.
It has to be remarked that the method according to the present invention achieves high clock accuracy in all parts of the network by distribution of clock reference signals rather than by implementing a high accuracy free running clock at each node. This synchronization is done hierarchically with a unique clock source of the hierarchy i.e. the Primary Reference Clock. A physical realization of a Primary Reference clock can be a cesium clock that has a frequency accuracy of some times 10xe2x88x9213. It can also be realized by a disciplined rhubidium controlled by a General Position System or a similar system.
It should further be noticed that the term xe2x80x9cincludingxe2x80x9d, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x9ca device including means A and Bxe2x80x9d should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noted that the term xe2x80x9ccoupledxe2x80x9d, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x9ca device A coupled to a device Bxe2x80x9d should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.